My 14 oz MSR water purifier (filter plus chlorox) weighs less than a liter of water and has saved my bacon a couple of times even while touring in the US and Canada. Of course, better planning my needs for water would have prevented these problems in the first place.

Even if you pull fom a tap, there is no assurance that the water you drink will not be contaminated. It is not crazy to filter any water than doesn't come from a bottle or a municipal water supply. In some places, I'd filter the water even then. Twice this year I found out too late that I had drunk from a tap connected to a contaminated well. Fortunately I didn't get sick. Some day they'll invent a filter that will work even though it stays in the pannier!